1. Field of the Invention
The present invention relates to a portable screen device for displaying an image projected from a projector, in particularly, to a portable screen device capable of moving a screen up and down when a screen is maintained horizontal by dual rotation shafts, maintaining the height of the screen by substituting a structure using a gas spring or a coil spring with a structure of right and left eccentric rotation shafts, having a remarkably reduced volume and weight, and increased stability.
2. Description of the Related Art
In portable screen devices, since the less a volume is the more convenient it is to carry and the lighter it is the more practical, a structure for reducing volume and weight is a very important development and goal.
As structures of a conventional portable screen device, a structure, in which a screen rolled around a spring rotation shaft is accommodated in an accommodating case, has been proposed and a structure, in which the accommodating case is installed at a certain place and a screen is pulled up or maintained at a height by a pantograph using elasticity of a coil spring or a gas spring, is already used.
As the conventional gas spring or the coil spring is used, the performance such as the quantity of gas is decreased or the elasticity of the coil spring is deteriorated. Actually, as a result of checking the screen devices returned by customers, malfunction caused by the coil spring or the gas spring accounts for more than 40 percent.
According to a movable screen device disclosed in Korean Patent Publication No. 10-2000-0017351 (Claimed Priority, Japanese Patent Application Nos. 98-234542 and 99-006345), rotation shafts of right and left arms of a pantograph are coaxially arranged by duplicating the arms at a top bar or at the lower end thereof in the front-to-rear direction to maintain their horizontality when the screen moves up and down.
The conventional movable screen device further includes a height-maintaining device for canceling the elastic force of a spring generated, when the screen moves up and down to a position, by the gas spring or the coil spring.
However, since two arms are duplicated in the front-to-rear direction about a vertical axis in the coaxial structure b, the volume of the conventional movable screen device is increased.
In addition, there is a plurality of the gas springs or the coil springs as the height-maintaining device, the conventional movable screen device is heavy and its volume is increased.
Moreover, the conventional movable screen device further includes a single lozenge-shaped member as a support assembly to form lozenge-shaped dual supports.
However, since in this movable screen device the right and the left arms are structured in the front-to-rear direction by the coaxial structure, its volume increases, and since the gas springs or the coils springs are uses as the height-maintaining device, the conventional movable screen device is also heavy and its volume increases.
Further, there is another conventional movable screen device in which a right elastic coupler and a left elastic coupler are installed to hinges for connecting the arms of the pantograph or the top bar such that the right and the left elastic couplers rotate in directions opposite to each other, plate springs installed in the rear sides of the right and the left elastic couplers to apply elastic force thereto, and a brake lever pushes the right and the left couplers. The another conventional movable screen device, like the conventional movable screen device, is identical to the conventional movable screen device in view of structuring the right and the left arm as the coaxial structure using a single shaft and has advantages capable of substituting the coil springs or the gas springs with frictional pads, spring devices, brake levers. However, the another conventional movable screen device rather increases its own weight and volume due to the additional devices such as the brake levers, the addition of elastic maintaining spring structure, and the expansion of a space necessary for operating the devices.
Moreover, according to the another conventional movable screen device, in order to fix the screen at a predetermined position, the brake levers must be released and must be locked at the predetermined position again sequentially so that the height of the screen is fixed. Thus, it is very inconvenient.